Mass spectrometry is an analytical methodology used for qualitative and quantitative determination of compounds in chemical and biological samples. Analytes in a sample are ionized, separated according to their mass by a spectrometer and detected to produce a mass spectrum. The mass spectrum provides information about the masses and in some cases the quantities of the various analytes that make up the sample. In particular embodiments, mass spectrometry can be used to determine the molecular weight or the molecular structure of an analyte in a sample. Because mass spectrometry is fast, specific and sensitive, mass spectrometer devices have been widely used for the rapid identification and characterization of biological analytes.
During the last few years, matrix-assisted laser desorption/ionization (MALDI) methods have proven to be valuable for the ionization of samples, and have found widespread use in a variety of fields such as genomics and proteomics. In performing MALDI methods, a sample is combined with an organic matrix that co-crystallizes with the sample, and then deposited upon a MALDI sample plate. The MALDI sample plate is placed in an MALDI ion source, and a laser beam vaporizes the sample. During vaporization of the sample, analyte ions are formed. It is thought that the presence of the matrix enables the analyte to be ionized, solving a problem of other methods. MALDI methods may be done at atmospheric pressure (e.g., in AP-MALDI) or at below atmospheric pressure (e.g., in a vacuum or at an intermediate pressure).
In many cases, MALDI ion sources are integrated with an analytical device, e.g., a mass spectrometer, for studying the MALDI-ionized analyte. For the most part, time-of-flight mass spectrometers (“TOF-MS”) are used for this purpose, however a variety of other mass spectrometers may also be used, including an ion cyclotron resonance spectrometer (e.g., a Fourier transform ion cyclotron mass resonance spectrometer), an ion trap mass spectrometer (e.g., a high-frequency quadrupole ion trap mass spectrometer), or a hybrid instrument (e.g., a quadrupole/time-of-flight mass spectrometer, Q-TOF).
In ionizing a sample using MALDI, it is generally desirable to view an area on a MALDI sample plate to ensure that a sample has been deposited onto the that area, and to ensure that the MALDI laser is actually going to impact the sample. In particular, there is a need for an imaging system that provides a detailed image of the sample, in particular an image that shows areas of analyte crystals.
The present invention meets this, and other, needs.